Our streets are not designed for safe cycling and our public transport infrastructure is poor, at best. So the only way we can improve safety for cyclists and pedestrians is to reduce speed limits for cars across the board.
The political will to improve road/pavement safety for pedestrians and cyclists is not there (there's a war on motorists, apparently?) so while cyclists have to (by law) use public roads then we have to reduce car speed limits to make it safe.
Most modern cars have cruise control that work at 20mph, so the argument that it's difficult to stick to 20mph is lazy and incorrect. Studies out of Wales say that it adds approximately 30 seconds per journey and one of the quotes in this article _quoted a woman who does not drive!_